Barry Cardiff

Performance improvement of 10Gb/s direct modulation OFDM by optical injection using monolithically integrated discrete mode lasers

Performance improvement of a directly modulated 10Gb/s OFDM system by optical injection is shown experimentally over differing fibre lengths. The modulation and optical injection is performed using monolithically integrated Discrete Mode lasers.

On Bit and Power Loading for OFDM Over SI-POF

A new methodology is proposed for the modeling of practical uncoded bit and power loading schemes for OFDM, based on the assumption of a uniform transmit power spectrum. The accuracy of this technique, both in terms of predicted maximum data rate and occupied bandwidth, is demonstrated analytically for an Intensity Modulated/Direct Detected (IM/DD) system over Step Index Plastic Optical Fibers (SI-POF). It is shown that both this new methodology and the well known water pouring theorem accurately predict the maximum achievable data rate in practical uncoded systems; however, it is also demonstrated that the new methodology is a better predictor of the bandwidth occupied by the transmit signal, thus providing an important analytical tool for system designers. The analysis presented is also experimentally verified using an IM/DD optical system employing a directly modulated Resonant Cavity Light Emitting Diode (RCLED) light source and several lengths of 1 mm SI-POF.

Narrow-linewidth discrete-mode laser diodes for coherent communication applications

Lasers with narrow-linewidth emission are a key component for higher-order modulation formats. We report on discrete-mode laser diodes designed for narrow-linewidth emission and demonstrate linewidths less than 80 kHz. Using these devices in quadrature phase shift keying and 16-quadrature amplitude modulation transmission setups, similar performance to that of an external cavity laser is demonstrated.

Increased Bit Rate Direct Modulation AMO-OFDM Transmission by Optical Injection Using Monolithically Integrated Lasers

Experimental and simulation work, presented in this letter, demonstrates for the first time how the monolithic integration of two single-mode lasers in a master-slave configuration, can substantially increase the achievable bit rate of a direct modulation adaptively modulated optical orthogonal frequency-division multiplexing (AMO-OFDM) system. The Levin-Campello algorithm is applied to select the OFDM bit and power loading scheme used for each system configuration. Improvement in terms of data throughput due to injection is measured for several transmission distances with the improvement in performance presented in terms of error vector magnitude per OFDM subcarrier.

Multiple Decision Feedback Equalizers for vector systems with complexity/performance tradeoff

We consider detectors for the class of vector systems describable by the matrix equation y = Ax + n, where y is a vector of observations, n the additive noise vector, A the known system matrix, and x are the unknown (to be estimated) symbols drawn from a finite symbol alphabet. A novel detector is developed consisting of the combination of Multiple Decision Feedback Equalizers (M-DFE) each with a permutation functionality included in such a way as to produce a detector structure that can significantly out-perform the conventional single DFE. Different arrangements are possible allowing a complexity / performance trade-off. The serial cascade where each DFE adapts its permutation based on reliability metrics from the previous DFE out-performs the parallel autonomous arrangement for the same number of DFEs. We also outline complexity reduction techniques for the special cases of Hadamard based precoded OFDM and single carrier OFDM that are more applicable to the parallel arrangement.

Narrow linewidth discrete mode laser diodes at 1550 nm

We report on discrete mode laser diodes designed for narrow linewidth emission and demonstrate a linewidth as low as 96 kHz. A discrete mode laser diode with a minimum linewidth of 189 kHz was also characterised in a coherent transmission setup using quadrature phase shift keying modulation. Similar performance to an external cavity laser is demonstrated at baud rates as low as 2.5 Gbaud. The effect of increased linewidth on transmission performance is also investigated using lasers with linewidths up to 1.5 MHz.

Reliability-Ordered DFE for vector systems

In this paper, detectors are considered for a general class of vector systems. A novel detector, the Reliability-Ordered Decision Feedback Equalizer (RO-DFE), is presented in which the ordering of decisions is done in accordance with reliability metrics helping to mitigate the problem of the error propagation inherent to all DFEs. The proposed structure derives the reliability metrics from an initial linear MMSE equalizer. This yields the major advantage that soft decisions from this equalizer can be reused within the DFE resulting in only a minor overall complexity increase. For an important class of channel independent OFDM pre-coders, the equivalence between a hypothetical detector and one operating on an equivalent AWGN channel is established. This is then used to derive a lower bound on the performance of a Maximum Likelihood (ML) detector in terms of Bit Error Rate for QPSK, and in terms of Symbol Error Rate for M-ary QAM over a maximum diversity Rayleigh fading channel. Simulation results are presented showing significant gains of the proposed detection system over conventional ones. The newly derived ML lower bound for QPSK is also compared to a simulated ML implementation and found to be accurate under good SNR conditions.

A Generic Foreground Calibration Algorithm For ADCs with Nonlinear Impairments

This paper presents a generic foreground calibration algorithm that estimates and corrects memoryless nonlinear impairments in both single channel and time-interleaved analog-to-digital converters (TIADCs), and which is capable of correcting for amplifier nonlinearity, comparator offsets, and capacitance mismatch for each channel. It operates by generating, and then using, a look-up table which maps raw ADC output decision vectors to linearized output. For TIADCs, the algorithm also uses information gained during the calibration phase to estimate timing and gain mismatches among the sub-ADCs. The problem of selecting an appropriate timing reference so as to relax the requirements on the time-skew correction circuitry is statistically analyzed, as is the corresponding impact on manufacturing yield. Accordingly, a new method is proposed having superior performance; for example, in the case of an eight sub-ADC TIADC system, the proposed scheme reduces the time skew correction requirement by 44% compared with conventional methods. The architecture is instrumented with some additional circuitry to facilitate built-in self-test, allowing manufacturing test time and cost reductions. Implementation aspects are discussed, and several complexity reduction techniques are presented along with synthesis results from a Verilog implementation of the calibration engine.

Words alignment based on association rules for cross-domain sentiment classification

Automatic classification of sentiment data (e.g., reviews, blogs) has many applications in enterprise user management systems, and can help us understand people’s attitudes about products or services. However, it is difficult to train an accurate sentiment classifier for different domains. One of the major reasons is that people often use different words to express the same sentiment in different domains, and we cannot easily find a direct mapping relationship between them to reduce the differences between domains. So, the accuracy of the sentiment classifier will decline sharply when we apply a classifier trained in one domain to other domains. In this paper, we propose a novel approach called words alignment based on association rules (WAAR) for cross-domain sentiment classification, which can establish an indirect mapping relationship between domain-specific words in different domains by learning the strong association rules between domain-shared words and domain-specific words in the same domain. In this way, the differences between the source domain and target domain can be reduced to some extent, and a more accurate cross-domain classifier can be trained. Experimental results on Amazon® datasets show the effectiveness of our approach on improving the performance of cross-domain sentiment classification.

A low-complexity correlation-based time skew estimation technique for time-interleaved SAR ADCs

This paper presents a technique to estimate the time skew in time-interleaved ADCs. The proposed method estimates all of the time skew parameters jointly based on observations from a bank of correlators. The proposed method works for an arbitrary number of sub-ADCs. For implementation of the correlator bank, we propose the use of Mitchells logarithmic multiplier and a hardware reuse mechanism, thereby reducing the complexity and power consumption. Also, we explain why blind estimation techniques alone (including the proposed one) are not always sufficient for time skew estimation for certain classes of input signal; for the proposed approach, however, a simple modification to the analogue circuit (suitable for SAR ADCs) is shown to successfully deal with such problems, with only a minor penalty in power and area. The technique is verified by extensive simulations including a spectrally rich input signal in which an MTPR (multi-tone power ratio) improvement from 29dB to 62dB was achieved for a TIADC system having 16 sub-ADCs.